1. Field of the Invention
The present invention relates to a synthetic calibrator and its use in a sandwich immunoassay.
2. Description of Related Art
On account of their particularly good specificity and sensitivity, immunoassays are frequently employed for detecting proteins, for the purposes of medical diagnosis, in serum samples or urine samples. This requires, in addition to one or two specific antibodies, a calibrator, which is used as a comparison standard for quantifying the patient samples. It is desirable to be able to store the calibrators at 4.degree. C. for periods of several weeks to months, particularly in the case of automated assays carried out in large analytical laboratories. Depending on the analyte, these demands placed on the stability of the calibrator formulation can give rise to difficulties if, for example, there is no guarantee of solubility under physiological salt and pH conditions. As an example, mention may be made in this context of troponin I and troponin T, which are only adequately stable and soluble in denaturing solutions (6 M urea, 0.01 M dithiothreitol). However, it is not possible to establish any immunoassay using this denaturing formulation, since the antibodies are damaged by this treatment.
It is known that proteins are relatively unstable in solution and that reagents containing them are frequently sold in freeze-dried form, together with a solvent of suitable composition in which the experimenter has to dissolve them prior to use. If the solutions which are obtained in this way are stored at 4.degree. C., they can be used for several days even if daily determination indicates that the concentration of the reagent is changing to some extent. In general, therefore, it is recommended--in the case of troponin I (TnI) and troponin T (TnT) as well--that the comparison solutions which are obtained from the freeze-dried material be frozen in unit-dose form if they are to be stored for a relatively long period.